Even parity $\Theta$ pentaquark and stable antistrange nuclear matter

The newly discovered exotic $\Theta$ baryon of mass $1540\mathrm{MeV}$ (and very small width) truly has a very low mass, if it is a pentaquark system of even parity. A schematic model in which the coherent interaction of $u\overline{s}$ and $d\overline{s}$ pairs leads to a very large residual attractive interaction is introduced. This collective vibrational model accounts for the mass and small decay width to the $KN$ channel, but yields a significant coupling to the virtual $K*N$ channel. The schematic model predicts an attractive $\Theta$-nucleon interaction sufficient to produce a $\Theta$-nuclear bound state stable against strong decays. The discovery of $\Theta$-nuclei could be a proof that the $\Theta$ parity is even.

Figure 1

Graphical solution of Eq. (6). The variable $x$ represents the quantity $M_{\Theta }-M$.

Figure 2

$\Theta N$ interaction. The heavy lines represents the $\Theta$ and the light one the nucleon. Coherent excitations in the $\Theta$ are denoted by the dashed line, $\widehat{V}$ is represented by the circle.